PROJECT SUMMARY/ABSTRACT Hemolytic diseases, including sickle cell disease (SCD), malaria, and sepsis, affect millions of people worldwide each year. Approximately one in 360 African Americans is diagnosed with SCD each year, an estimated 214 million cases of malaria are reported each year, and sepsis is the leading cause of death in intensive care units. Increased levels of circulating free heme (heme crisis) is a key characteristic of hemolytic disease that results in direct and indirect production of reactive oxygen species (ROS). The overproduction of ROS can cause overt endothelial and organ damage. Xanthine oxidase (XO) is one such enzymatic source of ROS that has been shown to be elevated in a number of hemolytic diseases including SCD (4-fold), malaria (5-fold), and sepsis (9- fold). XO is produced primarily in the liver, but under stress conditions such as hypoxia, ischemia, and inflammation XO can be released into circulation. Here XO can bind distal glycosaminoglycans on the apical surface of vascular endothelium and has the potential to generate ROS directly at the endothelial surface. While it has been shown that XO activity is increased in hemolytic disease, the role XO plays during heme crisis has not been well defined. The literature suggests increased XO activity is harmful via the generation of hydrogen peroxide (H2O2) and superoxide (O2?-) during the oxidation of hypoxanthine and xanthine in the final steps of the purine degradation pathway. However, our preliminary results suggest XO may instead have a protective role during severe heme crisis. We developed and validated a novel ?two hit? model of heme crisis in order to study XO?s mechanism of action in hemolytic disease. We examined the role of XO by using this model in combination with pharmacological inhibition of XO by the specific, FDA approved inhibitor febuxostat. Febuxostat pre-treated mice had a worsened survival rate compared to non-treated mice and showed accelerated organ damage and inflammatory response. Our preliminary data led to the formation of the following aims: 1) Establish the role of hepatic XO released into circulation in response to intravascular heme crisis, and 2) Determine if XO serves to degrade heme and protect against endothelial damage. To assess Aim 1 a liver-specific XO knockout mouse and an adeno-associated virus with an albumin promotor for liver-specific XO overexpression have been generated. The XO liver-specific knockout and overexpression mouse models will be challenged by heme crisis and their response will be characterized by 24-hour survival rate, organ damage, and endothelial damage. To assess Aim 2 a series of biochemical assays to measure changes in hemin absorbance, reaction kinetics, and release of free iron will be completed. In addition, the effects of potential heme degradation via XO on primary human pulmonary vascular endothelial cell permeability and viability will also be completed. Together, these aims will indicate whether XO has a protective role during heme crisis by serving as an additional source of heme degradation. Completion of these aims has a broad impact on a number of hemolytic diseases and has the potential to identify a novel target for treatment of heme crisis in diseases such as SCD, malaria, and sepsis.